... TGA importer.

Be more thorough on valid TGA RGB and RGBA images.
In particular current TGA plug-in can import RGBA as 32 bits (8 bits per
channel) and 16 bits (5 bits per color channel and 1 bit for alpha), and
RGB as 15 and 24 bits.
Maybe there exist more variants, but if they do exist, we simply don't
support them yet.

Thanks to Hanno Böck for the report and a first patch attempt.

(cherry picked from commit 674b62ad)